1. Field of the Invention
This invention relates to the removal of chromium values, present, generally, in the form of alkali metal or alkaline earth metal dichromates, from aqueous liquids such as electroplating wastewater but, particularly, electrolytically-produced aqueous alkali or alkaline earth metal chlorate solutions.
2. Description of the Prior Art
An aqueous solution of sodium chlorate and sodium chloride is conventionally produced by the electrolysis of aqueous sodium chloride in diaphragmless electrolytic cells. The extent of electrolysis is controlled to produce a product from the cell in which the sodium chlorate and sodium chloride have the desired ratio, usually in the range of about 1:1 to about 20:1 and preferably in the range of about 2:1 to about 15:1. The aqueous solution may be further processed to crystallize out the sodium chlorate for sale in crystal form for a variety of purposes, for example, in the production of chlorine dioxide for use in the bleaching of chemical cellulosic pulps by reduction in the presence of a strong mineral acid, usually, sulfuric acid, or the aqueous solution may be used directly for that purpose.
In the electrolysis of sodium chloride to form sodium chlorate, it is conventional to add chromates in the hexavalent state, usually in the form of sodium bichromate dihydrate, Na.sub.2 Cr.sub.2 O.sub.7bl .2 H.sub.2 O, to the electrolyte in the cell to improve significantly the current efficiency of the cell in the conversion of sodium chloride to sodium chlorate. The cell product, also known as "cell liquor", therefore, generally contains significant amounts of chromate ions.
It is desirable to remove chromate ions from the cell product before employment of the same in chlorine dioxide generation and it is desirable to recover chromate ion for reuse in the electrolytic cells. In addition, chromate ions are a toxic pollutant, so that environmental considerations require removal of the chromate ions where discharge of a product stream containing such ions may be effected. A number of prior proposals have been made for the removal of chromate ions from cell liquor. This invention also relates to a method for treating electroplating wastewater or other aqueous liquids containing hexavalent chromium ions as a contaminant. Electroplating wastewaters often bear large amounts of heavy metal contaminants such as copper, cadmium, nickel, and chromium. While these heavy metals readily form hydroxides or sulfides, with the notable exception of chromium, the removal of chromium generally requires an additional treatment step to reduce the chromium ions from the hexavalent to the trivalent state prior to precipitation.
Among the chemicals used in the treatment of wastewater for reducing hexavalent chromium ions to the trivalent state, it is known to use ferrous sulfate, sodium bisulfite, sulfur dioxide, and sodium sulfide. While these chemicals work well as reactants for reducing the hexavalent chromium ions to the trivalent state, the quantity of sludge produced by each of these reactants can vary drastically. Since it is no longer sufficient merely to produce clean water, the volume of sludge produced for disposal is nearly as important as the product quality. It is known from U.S. Pat. No. 4,705,639 that in the disclosed sodium sulfide/ferrous sulfate treatment of wastewater for chromium ion reduction, the rate of chromium ion reduction depends upon the pH of the wastewater. In this process the electroplating wastewater is adjusted to a pH of about 8 to 10, treated with sodium sulfide and, thereafter, treated with ferrous sulfate or ferrous chloride to reduce the hexavalent chromium ions to the trivalent state. A process for the removal of hexavalent chromium ions from wastewater is also disclosed in U.S. Pat. No. 4,260,491 in which an aqueous composition containing hexavalent chromium ions and a chelating agent for trivalent chromium ions is treated at about pH 5 with both (1) a known reducing agent for converting hexavalent chromium ions to trivalent chromium ions and (2) with ferric or aluminum chloride or sulfate. This treatment is effective in producing a precipitate of chromic hydroxide and ferrous hydroxide upon raising the pH to about 7.5 to about 10.
In the processes discussed above for the removal of dichromates from electroplating baths and other metal treatment solutions, chloride ion is, typically, absent from such metal treatment solutions and, accordingly, very low levels of chromium ions can be obtained in the treated plating baths by reduction of the chromium ions contained therein from the hexavalent state to the trivalent state followed by the precipitation of the trivalent chromium ions as hydrated chromic oxide. The problem of achieving low levels of chromium ions in liquid solutions containing chloride ions is made more difficult by the fact that hydrated chromic oxide has a solubility product higher by a factor of 10.sup.5 when in the presence of chloride-containing solutions.
A solution of sodium chlorate, prepared as a feed for a chlorine dioxide generator of the R-2, R-3, and SVP process type, produced by the electrolysis of sodium chloride, generally, has a chloride content of about 200 grams per liter. Low levels of bichromate ions are required in electrolytic cell liquors in cells for the production of alkali metal chlorates, particularly, sodium chlorate, in order to increase the current efficiency of the cells, for instance, from about 70% current efficiency to about 95% current efficiency. In addition, the presence of low levels of bichromate ion in electrolytic chlorate cells inhibits the formation of explosive mixtures of hydrogen and oxygen. It has been estimated that for every ton of sodium chlorate solution prepared in electrolytic chlorate cells, about 2-10 kilograms of sodium dichromate are present as a contaminant where no effort is made to remove the sodium dichromate.
Although the sodium dichromate has no effect on chlorine dioxide production by reduction of sodium chlorate utilizing the Rapson R-2 process, in the more modern chlorine dioxide processes in which methanol is used as the reducing agent, the presence of chromium ions reduces process efficiency by preventing the smooth operation of the chlorine dioxide generating process. That is, the presence of chromate ions inhibits the crystal growth of the sodium sesquisulfate by-product of the process and thus, makes more difficult its removal. In addition, the environmental impact of the discharge of chromium compounds in the pulp mill product is a serious environmental concern in view of the fact that a 100 thousand tons per annum sodium chlorate plant producing 10 percent of its product as cell liquor would discharge the equivalent of some 20-100 tons per annum of sodium dichromate.
There are a number of methods in the prior art for the removal of alkali and alkaline earth metal chromates from cell liquors. Prior art methods include reduction of the chromate to mixed chromous and chromic salts with the precipitation of insoluble hydroxides in processes in which reduction is effected by water soluble sulfides, hydrazine, hydroxylamine, sulfites, ion exchange techniques, precipitation to form barium chromate, and electrochemical means. Many of these processes present problems which make their use either uneconomic or otherwise undesirable. Some of these processes are discussed below.
In U.S. Pat. No. 4,086,150 and U.S. Pat. No. 3,843,769 there are described methods for removing hexavalent chromium ions from concentrated aqueous alkali metal chlorate solutions by treating such solutions with a water soluble sulfide. In the '769 patent, the concentrated aqueous alkali metal chlorate solutions are treated with a water soluble sulfide under acid conditions, especially at pHs below 4 and at temperatures above ambient. Thereafter, the treated mixture is neutralized and insoluble compounds in which chromium is present in the trivalent state are removed by filtration. In the '150 patent, the chlorate liquor containing compounds of chromium in the hexavalent state are treated at a temperature of about 40.degree. to 60.degree. C. with a water-soluble sulfide at an initial pH in the range of 7-8. Thereafter, a water-soluble ferrous salt is added to the treated solution and the pH is adjusted to obtain a final pH of about 1.8 to about 3.0 so as to convert the chromate to an insoluble trivalent chromium-containing material in which chromium is in the trivalent state.
In U.S. Pat. No. 4,699,701, a process is disclosed for the removal of chromium compounds from chlorate solutions utilizing the cathodic reduction of chromium ions in the hexavalent state to chromium ions in the trivalent state and the deposition of chromium ions in the trivalent state on the cathode as chromium hydroxide.
The removal of hexavalent chromium ions in the hexavalent state from chromate cell liquor by ion exchange processes is described in U.S. Pat. No. 4,376,099 and U.S. Pat. No. 3,835,001. The use of hydrazine to selectively remove chromate ions from a chlorate solution is described in U.S. Pat. No. 4,268,486, Canadian 1,139,080, Canadian 1,119,772, and U.S. Pat. No. 4,367,213.
The use of ammonia or an amine as a means of reducing chromium ions in the hexavalent state to divalent or trivalent state chromium ions is disclosed in several prior art patents. In U.S. Pat. No. 4,804,528, there is described the use of a two-stage process in which ammonia is added to a chlorate-rich solution containing hypochlorite and dichromate ions produced by the electrolysis of brine. Under controlled conditions of pH and temperature, a mixed divalent and trivalent state chromium compound, i.e., chromium hydroxide, is precipitated from solution and removed. In this process ammonia is added to a chlorate cell liquor at a pH above 8.5 and at a temperature greater than 60.degree. C. Preferably, the pH of the solution during this reaction is above approximately 10.5. The reaction takes place over a period of about 30 minutes at a temperature of 80.degree. C. The precipitate of chromium hydroxide is removed by filtration.
In U.S. Pat. No. 4,446,026, there is disclosed a process for the extraction of chromates from an aqueous solution, for instance, from a sodium chlorate liquor produced by the electrolysis of brine, by contacting the aqueous chlorate cell liquor with an organic phase consisting of a tertiary or a quaternary amine so as to extract the chromate compounds. In this process, the reaction is conducted at a pH lower than 6 and at a temperature of about 35.degree. C.
In U.S. Pat. No. 4,956,154, a process is disclosed for the extraction of chromium ions from aqueous product solutions utilizing an extractant comprising in organophosphinic acid, a di-2-ethylhexyl phosphoric acid and/or an aliphatic amine to selectively separate chromium ions and other contaminating metal ions. The process is conducted at a pH of about 6.5 to 7.5.
In addition to the use of hydrazine as a reducing agent for the removal of compounds containing chromium in the hexavalent state from chlorate cell liquors, as illustrated in Canadian Patent 1,139,080 and Canadian Patent 1,119,772, hydroxylamine or addition compounds thereof such as hydroxylamine hydrochloride have been disclosed in the prior art as reducing agents for the removal of chromium compounds containing chromium in the hexavalent state from electroplating baths, electrolytic and non-electrolytic cleaning baths and from chlorate cell liquors. In Swedish Patent 218,495, a reducing agent selected from the group consisting hydrazine or a hydroxylamine, or an addition compound or salt thereof is utilized as a reducing agent in an electroplating bath, also containing copper and nickel ions, for the removal of chromium compounds containing chromium in the hexavalent state from plating baths and cleaning baths. The presence of hexavalent chromium ions in such baths leads to inferior quality deposits of copper and nickel from these baths. In German 3,032,131, a process is disclosed for the removal of dichromates from chlorate solutions utilizing hydroxylamine hydrochloride as a reducing agent at a concentration of at least 4 mol per mol of dichromate. The process is conducted at a temperature not under about 40.degree. C. and at a pH over 7. Preferably, the pH of the reactant solution is about 8 so as to cause the compounds produced containing chromium in the trivalent state to precipitate in the form of chromic hydroxide. Where the pH of the reactant solution is acidic prior to treatment with hydroxylamine hydrochloride, the pH can be subsequently adjusted to the desired value so as to cause the precipitation of chromic hydroxide.
In the process of the instant invention, improved efficiency of removal of compounds containing chromium in the hexavalent state can be obtained by conducting the reaction at elevated temperature and under neutral or acid pH conditions.